For the past several decades, feature scaling has been a driving force in the production of integrated circuits in the semiconductor industry. Scaling features to smaller and smaller size can enable the production of devices that include a larger number of functional units within the limited real estate of a semiconductor chip. For example, shrinking transistor size may allow for an increased number of memory devices to be placed within a given area of a semiconductor chip, leading to the production of memory devices with increased storage capacity. However, shrinking feature size can also lead to challenges during manufacturing that in some instances can be difficult to address.
With the foregoing in mind, spin transfer torque memory (STTM) is a type of memory device that is becoming of increasing interest in the semiconductor industry, due to the relatively small size of its elements, its potential for low power operation, and its potential for direct integration directly with other elements on a semiconductor chip, such as transistors.
Generally, the operation of STTM devices is predicated on the phenomenon of spin transfer torque. When a current is passed through a magnetization layer of such devices, called the fixed magnetic layer, the current will come out spin polarized. With the passing of each electron in the current through the fixed magnetic layer, the resulting spin (angular momentum) may be transferred to the magnetization of another magnetic layer in the device, called the free magnetic layer, resulting in a small change in the magnetization of the free magnetic layer. In effect, this is a torque which causes precession of the magnetization of the free magnetic layer. Likewise, a torque may be applied to an associated fixed magnetic layer, e.g., due to the reflection of electrons.
Ultimately, when an applied current (e.g., a pulse) exceeds a threshold value (which may be defined at least in part by damping caused by the magnetic material and its environment) the orientation of the magnetization of the free magnetic layer may be switched between a state that is parallel with the orientation of the magnetization of the fixed magnetic layer, and a state that is antiparallel with the orientation of the magnetization of the fixed magnetic layer. The orientation of the magnetization of the fixed magnetic layer may remain unchanged by the applied current, e.g., because the applied current is below a threshold for the fixed magnetic layer and/or because the orientation of the magnetization of the fixed magnetic layer may be “pinned” by one or more adjacent layers, such as a synthetic antiferromagnetic layer. As such, spin transfer torque can be used to flip the active elements in a random access memory, such as an STTM device.
Although previously developed STTM devices have proven useful, challenges have arisen as such devices have been scaled to smaller and smaller dimensions.